The present invention concerns an element of a screw compressor injected with water containing two co-operating rotors which are bearing-mounted in a housing, whereby this housing limits a rotor chamber in which the rotors are situated and in which flows out a water circuit for the injection of water, and which is provided with an inlet and an outlet and whereby the rotors are supported by means of axle journals, both on the side of the inlet and on the side of the outlet, on radial hydrodynamic slide bearings lubricated with water, and are supported also axially on the outlet side, and whereby, on the inlet side, opposite to the crosscut ends of the axle journals, is formed at least one chamber.
In such compressor elements which are injected with water, water is used as a lubricant instead of oil, for the rotors as well as their bearings.
Onto this water can be added additives such as an anti-corrosion agent and/or an agent which causes a depression of the freezing point.
This makes it possible to obtain oil-free compressed air in a simple manner and to cool the rotors, so that the compression temperature can be kept under control and the efficiency of the compression is large on the one hand, and to avoid sealing problems which would arise if the bearings were lubricated with oil, since water may not penetrate in such bearings and no oil may leak in the compressed air on the other hand.
These compressor elements contain hydrodynamic slide bearings for the radial positioning and hydrostatic or hydrodynamic slide bearings for the axial positioning of the rotors, as opposed to oil-lubricated compressors, which usually make use of rolling bearings.
The axial slide bearings, onto which water is added, have to absorb the axial force, exerted on the rotors by the compressed gas.
Such a compressor element is described in WO 99/13224. On the inlet side, opposite to each of the crosscut ends of the axle journals is formed a chamber, onto which is connected a discharge pipe which opens into the rotor chamber, not far from the inlet.
The chambers opposite to the crosscut ends of the axle journals collect the aqueous lubricating liquid coming from the radial bearings via restricting elements, and they are under a limited pressure.
Moreover, also on the inlet side, opposite to the axle journals or to rings fixed on these axle journals, are formed spaces, onto which is connected a discharge pipe in the same manner which communicates with the rotor chamber in the vicinity of the inlet.
Consequently, the axial forces on each rotor have to be absorbed almost exclusively by the axial bearing on the outlet side, which axial bearing is a combined hydrodynamic/hydrostatic bearing.
As the diameters of the axial bearings are restricted by the centre distance between the rotors, the magnitude of the reactive force which can be generated in the bearing will be determined by the water pressure in the bearing.
In the case of hydrostatic axial bearings, the feeding pressure, required to absorb the above-mentioned axial force, will be larger than the outlet pressure of the compressor element, and with such bearings, an extra pump is required to increase the feeding pressure of the water for the hydrostatic bearings.
In the case of hydrodynamic axial bearings, the speed must be sufficiently high in order to be able to build up enough hydrodynamic pressure, which makes starting up against the pressure impossible on the one hand, and which strongly restricts the magnitude of velocity and thus the field of action of the compressor.
As in the compressor element according to WO 99/13224 the axial bearings on the outlet side are combined hydrodynamic/hydrostatic bearings, the above-mentioned disadvantages are somewhat reduced, but in practice it appears that a pump is necessary to feed the axial bearings, and the compressor element cannot work under high pressures.
The invention aims an element of a screw compressor injected with water with water-lubricated bearings which does not have the above-mentioned disadvantages and consequently permits a more efficient bearing, whereby, as a result, no pump is required to feed the hydrostatic bearings on the one hand, and, in the case of hydrodynamic axial bearings, the compressor element has a larger field of action on the other hand.
This aim is reached according to the invention in that, only on the inlet side, the chamber which is formed opposite to the crosscut ends of the axle journals, is directly connected to a source of fluid under a pressure which is equal to at least 70% of the outlet pressure of the compressor element.
Thanks to the pressure in the chamber or chambers opposite to the crosscut ends on the inlet side, an axial pressure is created on the crosscut ends of the axle journals towards the outlet side which counteracts the axial force, exerted by the compressed gas on the rotors.
Preferably, a chamber is formed on the inlet side, opposite to each axle journal, and each chamber is directly connected to a source of a fluid under a pressure which is equal to at least 70% of the outlet pressure of the compressor element.
The chamber opposite to the crosscut ends of the axle journals on the inlet side can be connected to the part of the water circuit in which practically prevails the outlet pressure of the compressor element, so that the fluid is the injection water for the rotors.
According to another embodiment of the invention, the above-mentioned chamber is connected to the inside of the rotor chamber.
In this case, not only water but a mixture of gas and water is provided to the chamber. This chamber is preferably connected to the rotor chamber by means of a conduit which is connected to the wall of the rotor chamber in such a place that a mixture of gas and water will flow via the conduit, which still contains relatively much water.
The axial bearing of the axle journals on the outlet side can be formed by hydrodynamic slide bearings which are also connected to the part of the water circuit which is practically situated on the outlet pressure, so that also with such slide bearings the water supply is simple.
The axial bearing of the axle journals on the outlet side can also be formed of hydrostatic bearings which each contain a ring surrounding the axle journal and which is connected to a radially protruding collar on the side of the bodies of the rotors, with on either side in the housing a ring-shaped chamber filled with water under pressure which is connected to the part of the water circuit in which the outlet pressure practically prevails.
Preferably, the outlet of the compressor element opens into a water separator, and the part of the water circuit which is practically situated on the outlet pressure is a conduit which is connected to the water collector part of said water separator.
The compressor element can be driven via the outlet side.